Resorbable magnesium alloy

ABSTRACT

A magnesium alloy shown includes magnesium as a principal ingredient, and 0.5 to 8.0 percent ytterbium; 0.1 to 2.0 percent calcium; and 0.2 to 6.0 percent zinc, percentages calculated by weight. The magnesium alloy may be employed as an implant. Examples of implants include a plate, specifically a bone plate, a screw, a nail, a bone nail, a stent, a rod. Implants made of the specified alloy are suitable for implantation in animal or human body.

RELATED APPLICATION

This patent application claims the priority benefit of U.S. Provisional Patent Application Ser. No. 61/059,370 filed Jun. 6, 2008 and entitled “RESORBABLE MAGNESIUM ALLOY”, the content of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

Inventive subject matter described herein relates to a magnesium alloy, implants made from the alloy, methods of manufacture and methods of use of the alloy.

BACKGROUND

In modem medical technology, implants are used for a broad range of applications, for example for orthopedic purposes, as support for vessels, and for attaching or fixing tissues or bones. Often the implants have only a temporary function until completion of the healing process. In order to avoid complications resulting from these implants remaining permanently in the body, they often must be operatively removed or made of a biocorrodible material which will be gradually degraded by the body. A growing number of such biocorrodible materials based on polymers or alloys are known. Of special interest are alloys made of biodegradable metals such as magnesium, iron and tungsten.

European Patent 1 270 023 describes a magnesium alloy which is reportedly suitable for the manufacture of endovascular or orthopedic implants. The alloy contains more than 50% magnesium and up to 5% rare earth metals. Other elements like aluminum, lithium and iron may also be contained in the disclosed alloy.

Application WO2008/035948 describes a biodegradable magnesium based alloy, comprising up to 40 atomic percent calcium as well as up to 40 atomic percent of one or more trace elements. The disclosed trace elements include Zr, Mo, Nb, Ta, Ti, Sr, Cr, Mn, Zn, Si, P, Ni and Fe. Through the addition of the trace elements the degradation rate of the magnesium alloy is reportedly varied.

Biodegradable magnesium alloys are also known to contain yttrium. WO02/100452 describes an alloy optionally comprising 0.01 to 7% by weight of yttrium and 0.01 to 8% by weight of rare earth metals. The alloy may also contain lithium and/or aluminum.

Ytterbium has been used as radiopaque marker element in implants. US2008/0033530 describes a marker alloy comprising 40 to 90 atomic percent of ytterbium, as well as 10 to 60 atomic percent magnesium and 0 to 10 atomic percent of one or several elements selected from the group of Ag, Zn, Au, Ga, Pd, Pt, Al, Sn, Ca, Nd, Ba, Si, and Ge. The alloy thus has a sufficient X-ray density at low material thicknesses. The degradation of ytterbium is reported as being approximately equal to the degradation of the main body.

Patent Application JP2004099940 describes a lightweight magnesium based alloy reportedly combining high strength with high ductility. The composition of the alloy comprises 0.5 to 5 atomic percent rare earth elements selected from Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and miscl. metal. The alloy further comprises 0.2 to 4 atomic percent zinc.

SUMMARY

Inventive subject matter described herein includes a biocorrodible and age-hardenable alloy, comprised essentially of magnesium, ytterbium, zinc and calcium.

Inventive subject matter also includes an implant incorporating an age-hardenable magnesium alloy, comprising ytterbium, zinc and calcium.

DETAILED DESCRIPTION

According to one exemplary embodiment, the magnesium alloy includes ytterbium, calcium and zinc. The three elements are present in following amounts:

-   -   Ytterbium: 0.5 to 8.0 weight percent     -   Zinc: 0.2 to 6.0 weight percent     -   Calcium: 0.1 to 2.0 weight percent         The balance up to 100 weight percent includes magnesium as well         as unavoidable impurities. For example, such impurities may stem         from the production process of the alloy or from impurities         already contained in the source material.

Surprisingly it was found that ytterbium containing magnesium alloys show a significantly increased age-hardenability if zinc and calcium are present. Moreover, such alloy embodiments also exhibit favorable corrosion properties in chlorine containing aqueous environments.

This alloy is produced, in some embodiments, by a micro-alloying process embodiment, such as is conventionally known. The micro-alloying process in combination with the chosen elements of embodiments described herein, which have a high grain growth restriction factor, enables the manufacture of an alloy having very good cold-forming properties and a low mechanical anisotropy. Composition described herein induce a fine grained structure in the alloy during solidification as well as during subsequent hot-forming processes. This is mainly due to the formation of fine precipitations of these elements which restrict undesirable grain growth during recrystallization.

According to another exemplary embodiment, the ytterbium containing magnesium alloy as described herein may contain further elements, that include manganese, zirconium, aluminum and elements from the group of rare earths. From this group, scandium and yttrium are used in some embodiments. The amount of each of these additional elements in the alloy is up to 4.0 weight percent, for some embodiments, up to 3.0 weight percent for some embodiments, up to 2.0 weight percent for some embodiments, and up to 1.0 weight percent for some embodiments. The skilled artisan understands that any combination of these elements may be added, each of which makes up to 4.0 weight percent, for some embodiments up to 3.0 weight percent, for some embodiments up to 2.0 weight percent, and for some embodiments up to 1.0 weight percent of the total weight of the alloy. The collective term “rare earths” as used herein includes scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. It should be understood that additional elements in the alloy are not limited to those listed above, but that other elements, especially from the group of metals and transition metals, may be added as well.

An implant containing the alloy as disclosed herein may be produced using techniques known in the art. The implant may be in any form, especially in the form of a plate, specifically a bone plate, a screw, a nail, a bone nail, a stent, a rod. Implants made of the specified alloy are suitable for implantation in animal or human body.

The foregoing description of embodiments has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. It is intended that the scope of the invention be defined by the following claims and their equivalents.

EXAMPLE

Novel magnesium alloys containing 4 wt-% Yb, 0.8 wt-% Zn and 0.25 wt % Ca were melted and cast in an induction furnace in Ar-atmosphere. The billets were extruded at a temperature of 350° C. to an end-diameter of 8.6 mm, which corresponds to an extrusion-ratio of 12.5. The microstructure of the extruded alloys showed a very fine-grained structure with a grain size of approximately 5 μm. This material featured an average yield strength of 150 MPa, tensile strength of 250 MPa, uniform elongation of 20% and elongation to fracture of 28%. Hardness measurements indicated an age hardening response, where the values increased from approximately 50 HV5 in the solution heat-treated state to approximately 70 HV5 in the age hardened state. 

1. A magnesium alloy, comprising, by percentage of weight: 0.5 to 8.0 percent ytterbium 0.1 to 2.0 percent calcium 0.2 to 6.0 percent zinc wherein the balance up to 100% by weight is magnesium.
 2. The alloy of claim 1, further comprising, as an alloy element, up to 4.0 percent by weight scandium.
 3. The alloy of claim 1, further comprising, as an alloy element, up to 2.0 percent by weight yttrium.
 4. The alloy of claim 1, further comprising, as an alloy element, 0.05 to 1.0 percent by weight manganese.
 5. The alloy of claim 1, further comprising, as an alloy element, up to 1.0 percent by weight zirconium.
 6. The alloy of claim 1, further comprising, as an alloy element, up to 2.0 percent by weight aluminum.
 7. An implant comprising the magnesium alloy of claim
 1. 8. The alloy of claim 1, wherein the alloy has a grain structure with grains of approximately 5 μm.
 9. The alloy of claim 1, wherein the alloy has an age hardened hardness value of approximately 70 on a Vickers HV5 scale. 